States of Matter. Changes in State

Transcription

1 CHAPTER 8 States of Matter LESSON 2 Changes in State What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 3. Particles of matter have both potential energy and kinetic energy. 4. When a solid melts, thermal energy is removed from the solid. Key Concepts How is temperature related to particle motion? How are temperature and thermal energy different? What happens to thermal energy when matter changes from one state to another? Kinetic and Potential Energy When snow melts after a snowstorm, all three states of water are present. The snow is a solid, the melted snow is a liquid, and the air above the snow and ice contains water vapor, a gas. What causes particles to change state? Kinetic Energy Recall that the particles that make up matter are always moving. These particles have kinetic energy, the energy an object has due to its motion. The faster particles move, the more kinetic energy they have. Within a given substance, such as water, particles in the solid state have the least amount of kinetic energy. This is because they only vibrate in place. Particles in the liquid state move faster than particles in the solid state. Therefore, they have more kinetic energy. Particles in the gaseous state move quickly. They have the most kinetic energy of particles of a given substance. Temperature is a measure of the average kinetic energy of all the particles in an object. Within a given substance, a rise in temperature means that the particles, on average, are moving at greater speeds. Therefore, the particles have more kinetic energy. For example, water molecules at 25 C are moving faster and have more kinetic energy than water molecules at 10 C. Building Vocabulary Skim this lesson and circle any words you do not know. If you still do not understand a word after reading the lesson, look it up in the dictionary. Keep a list of these words and definitions to refer to when you study other chapters. Key Concept Check 1. Relate How is temperature related to particle motion? Reading Essentials States of Matter 143

2 Reading Check 2. Apply Which has more potential energy: a baseball held 1 m above the ground or a baseball held 2 m above the ground? 3. Model Imagine two balls connected by a spring. In which arrangement do the balls have more potential energy: when they are pulled apart or when they are closer together? Key Concept Check 4. Contrast How do thermal energy and temperature differ? Potential Energy In addition to kinetic energy, particles have potential energy. Recall that potential energy is stored energy due to the interactions between particles or objects. Think about holding a basketball and then letting it go. The gravitational force between the ball and Earth causes the ball to fall toward Earth. Before you let the ball go, it has potential, or stored, energy. Potential energy typically increases when objects get farther apart. It decreases when objects get closer together. When you hold up a basketball, it is farther off the ground than when it is falling from your hands. It has a higher potential energy than when it is falling. When the basketball is touching the ground, it has no more potential energy. The farther an object is from Earth s surface, the greater its gravitational potential energy is. As the ball gets closer to the ground, its potential energy decreases. You can think of the potential energy of particles in a similar way. The chemical potential energy of particles is due to their position relative to other particles. The chemical potential energy of particles increases and decreases as the distances between particles increase or decrease. Thus, particles that are farther apart have greater chemical potential energy than particles that are closer together. Thermal Energy Changes in state are caused by changes in thermal energy. Thermal energy is the total potential and kinetic energies of an object. You can change an object s state of matter by adding or removing thermal energy. When you add thermal energy to an object, these things can happen: Particles move faster (increased kinetic energy). Particles get farther apart (increased potential energy). Particles get faster and move farther apart (increased kinetic and potential energy). The opposite is true when you remove thermal energy: Particles move slower (less kinetic energy). Particles get closer together (less potential energy). Particles move slower and closer together (less kinetic and potential energy). If enough thermal energy is added or removed, a change of state can occur. 144 States of Matter Reading Essentials

3 Solid to Liquid or Liquid to Solid After you drink a beverage from an aluminum can, do you recycle the can? Aluminum recycling is one example of a process that involves matter changing from one state to another by adding or removing thermal energy. Melting The first part of the recycling process involves melting aluminum cans. To change matter from a solid to a liquid, thermal energy must be added. The graph below shows the relationship between increasing temperature and increasing thermal energy (potential energy + kinetic energy). At first, the thermal energy and the temperature increase. The temperature stops rising when it reaches the melting point of the matter. The melting point is the temperature at which the solid changes to a liquid. As aluminum changes from solid to liquid, the temperature does not change. However, energy changes still occur. Energy Changes What happens when a solid reaches its melting point? Notice that the line on the graph below is horizontal. This means that the temperature, or average kinetic energy, stops increasing. However, the amount of thermal energy continues to increase. How is this possible? Once a solid reaches the melting point, additional thermal energy causes the particles to overcome their attractive forces. The particles move farther apart and potential energy increases. Once a solid completely melts, the addition of thermal energy will cause the kinetic energy of the particles to increase again, as shown by a temperature increase. Temperature Increases Reading Check 5. Infer What must be added to matter to change it from a solid to a liquid? 6. Analyze During melting, which factor remains constant? Thermal Energy v. Temperature as Solid Changes to Liquid Solid Melting T constant PE increases Liquid T = temperature (average kinetic energy) PE = potential energy Thermal Energy Increases Reading Essentials States of Matter 145

4 Reading Check 7. Define freezing. 8. Explain Why doesn t the evaporation flask have bubbles below the surface? During evaporation, a liquid vaporizes only at its surface. Freezing After the aluminum melts, it is poured into molds to cool. As the aluminum cools, thermal energy leaves it. If enough energy is removed, the aluminum will freeze. Freezing is a process that is the opposite of melting liquid changes to solid. The temperature at which matter changes from the liquid state to the solid state is its freezing point. You can look at the graph of melting on the previous page to follow the process of freezing as thermal energy is removed. To observe the temperature and thermal energy changes that take place as liquid aluminum forms solid blocks, move from right to left on the graph on the previous page. Liquid to Gas or Gas to Liquid When you heat water, do you ever notice how bubbles begin to form at the bottom and rise to the surface? The bubbles contain water vapor, a gas. As the water heats, it changes from the liquid state to the gaseous state. The change in state of a liquid into a gas is vaporization. The figure below shows two types of vaporization evaporation and boiling. The two types of vaporization differ in where they take place in the liquid. Boiling Vaporization that occurs within a liquid is called boiling. During boiling, vaporization takes place throughout the liquid. The temperature at which boiling occurs in a liquid is called its boiling point. Boiling and Evaporation During boiling, a liquid vaporizes at its surface and within the liquid. Bubbles, or vaporized particles, rise to the top of the liquid and escape from the container. 146 States of Matter Reading Essentials

5 Temperature Increases Liquid Thermal Energy v. Temperature as Liquid Changes to Gas Boiling T constant PE increases Thermal Energy Increases Gas T = temperature (average kinetic energy) PE = potential energy 9. Explain Why does the liquid change to a gas? In the graph above, notice the energy changes that occur as thermal energy is added. The kinetic energy of particles increases until the liquid reaches its boiling point. At the boiling point, the potential energy of particles begins increasing. The particles move farther apart until the attractive forces no longer hold them together. At this point, the liquid changes to a gas. When boiling ends, if thermal energy continues to be added, the kinetic energy of the gas particles begins to increase again. Therefore, the temperature begins to increase again as shown on the graph above. Evaporation Unlike boiling, evaporation is vaporization that occurs only at the surface of a liquid. A small amount of liquid in an open container will disappear after several days due to evaporation. Condensation Boiling and evaporation are processes that change a liquid to a gas. The opposite process also occurs. When a gas loses enough thermal energy, the gas changes to a liquid, or condenses. The change of state from a gas to a liquid is called condensation. Overnight, water vapor often condenses on blades of grass and forms dew. Solid to Gas or Gas to Solid A solid can become a gas without turning into a liquid. Also, a gas can become a solid without turning into a liquid. Solid to Gas Dry ice is solid carbon dioxide. It turns immediately into a gas when thermal energy is added to it. The process is called sublimation. Sublimation is the change of state from a solid to a gas without going through the liquid state. As dry ice sublimes, it cools and condenses the water vapor in the surrounding air, creating a thick fog. Make a four-tab Foldable and record what you learn about each term under the tabs. Vaporization Boiling Evaporation Condensation Sublimation Deposition 10. Apply Clouds can form when water vapor in the air condenses. Clouds are what state of matter? Reading Essentials States of Matter 147

6 Reading Check 11. Evaluate Why are sublimation and deposition unusual changes of state? Key Concept Check 12. Describe the changes in thermal energy as water goes from a solid to a liquid. 13. Locate Circle the location on the graph where water reaches its boiling point. Temperature 100 C 0 C Melting T constant PE increases Solid Gas to Solid The opposite of sublimation is deposition. Deposition is the change of state of a gas to a solid without going through the liquid state. For deposition to happen, thermal energy must be removed from the gas. Frost on grass on a fall morning is often the result of deposition. As water vapor loses thermal energy, it changes into solid frost. States of Water Water is the only substance that exists naturally as a solid, a liquid, and a gas within Earth s temperature range. To better understand the energy changes during a change in state, look at the heating curve of water shown in the graph below. Adding Thermal Energy Suppose you place a beaker of ice on a hot plate. The hot plate moves thermal energy to the beaker and the ice. The temperature of the ice increases. Recall that this means the average kinetic energy of the water molecules increases. At 0 C, the melting point of water, the water molecules vibrate so rapidly that they begin to move out of their places. At this point, any added thermal energy causes the particles to overcome their attractive forces, and melting occurs. Once melting is complete, the kinetic energy of the particles begins to increase again as more thermal energy is added. Then the temperature begins to increase, too. When water reaches 100 C, its boiling point, liquid water begins to change to water vapor. Again, kinetic energy stays the same as vaporization occurs. When the change of state is complete, the kinetic energy of molecules increases again, and so does the temperature. The Heating Curve of Water Liquid Thermal Energy Increases Boiling T constant PE increases Gas T = temperature (average kinetic energy) PE = potential energy 148 States of Matter Reading Essentials

7 Removing Thermal Energy The removal of thermal energy is the reverse of the process shown in the heating curve of water. You can follow what happens to water vapor as you remove thermal energy by following the graph on the previous page from right to left. Cooling water vapor changes the gas to a liquid. Cooling the water further changes it to ice. Conservation of Mass and Energy The diagram below shows the energy changes that take place as thermal energy is added or removed from matter. Notice that there are three sets of opposite processes: melting and freezing vaporization and condensation sublimation and deposition During all of these changes of state, matter and energy are always conserved. Sometimes, such as when water vaporizes, it seems to have disappeared. However, it has just formed an invisible gas. If the gas were captured and its mass added to the remaining mass of the liquid, you would see that matter is conserved. The same is true for energy. Surrounding matter often absorbs thermal energy. If you measured thermal energy in the matter and the surrounding matter, you would find that energy is also conserved. Solid Melting add thermal energy Freezing remove thermal energy Changes of State Sublimation add thermal energy Liquid Deposition remove thermal energy Reading Check 14. Describe what happens to water vapor when thermal energy is removed from it. 15. Draw Circle the state of matter that results when thermal energy is added to a liquid. Vaporization add thermal energy Condensation remove thermal energy Gas Reading Essentials States of Matter 149

8 Mini Glossary condensation: the change of state from a gas to a liquid deposition: the change of state of a gas to a solid without going through the liquid state evaporation: vaporization that occurs only at the surface of a liquid kinetic energy: the energy an object has due to its motion sublimation: the change of state from a solid to a gas without going through the liquid state temperature: a measure of the average kinetic energy of all the particles in an object thermal energy: the total potential and kinetic energies of an object vaporization: the change in state of a liquid into a gas 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that includes an example of one change of state and its opposite process. 2. Write the correct term next to its opposite term in each row of the table below. Happens When Thermal Energy Is Added Happens When Thermal Energy Is Removed melting sublimation 3. Name two types of vaporization. How are they different? What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED condensation Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 150 States of Matter Reading Essentials